What is in Sanskrit grammar, a lengthening of the simple vowels a, i, e, by prefixing an a element. The term is sometimes used to denote the same vowel change in other languages called?

guna — In Sanskrit grammar, a lengthening of the simple vowels a, i, e, by prefixing an a element. The term is sometimes used to denote the same vowel change in other languages. (Webster's 1913.)